When the engine is stopped after a period of operation during which the temperature of the engine, e.g. coolant, oil and material, has increased above a certain level, and relatively shortly after that started again (“hot” start), the first couple of combustions can be in the form of knocking when the start procedure is the same as during a cold engine. In brief, the reason is that when the engine is stopped, it will heat the air in the inlet manifold, and a high inlet air temperature and pressure combined with a low engine speed at restart can easily result in knocking. The high inlet pressure is due to the pressure reaching atmospheric pressure when the engine stops.
Accordingly, the present invention is directed to improving performance of an internal combustion engine with spark ignition at a starting procedure of the engine, and in particular to reducing the knocking in an internal combustion engine in a vehicle, while still providing a desirable behavior from a user/driver point of view during a “hot” start.
The method for improving performance of an internal combustion engine with at least one cylinder, at each of which at least one spark providing device for ignition being provided includes: in connection to a start procedure of the engine, operating the engine in a controlled self-ignition mode comprising at least one controlled combustion without spark ignition.
The provision of at least one spark providing device, for example a spark plug, at each cylinder, means that the engine can be operated in a spark ignition mode.
The invention solves the problem of knocking when starting a hot engine in an elegant and simple manner. The inventive idea can be viewed as instead of attempting to eliminate the pre-ignitions at a “hot” start, the pre-ignition behavior is “transformed” into cycles with controlled combustion without spark ignition. This means that there is no flame propagation combustion. Instead, auto-ignition is controlled to an extent that no engine knock will occur. The invention will eliminate the problem of avoiding knocking after engine start during “hot” conditions, fuel qualities and engine crank speed at start, but still the invention will provide an engine than provides torque quickly at the startup phase. In other words, the number of cranks without combustion is reduced, which is advantageous from a user/driver point of view in case the engine is provided in a vehicle.
In an exemplary embodiment, the controlled combustion without spark ignition comprises homogeneous charge compression ignition (HCCI), or controlled auto-ignition (CAI). However, controlled combustion without spark ignition does not mean that the fuel/air mixture has to be totally homogeneous. The mixture can in certain embodiments be more or less stratified, for example if direct fuel injection is used. As explained further below, the invention can be used in combination with any fuel injection system.
In case of HCCI cycles, in order to achieve them, the homogeneous air-fuel mixture can be created either in the inlet manifold or in the cylinder by early fuel injection and fast fuel-air mixing. A homogeneous air-fuel mixture in the inlet manifold may be achieved, as in the case of a conventional Otto cycle engine, by using a fuel-aspirating carburetor or by using a low-pressure fuel injection pump and nozzle. It should be noted that the HCCI mode gives reduced levels of nitrous oxide (NOx) in the exhaust gases, due to the low combustion temperature of the diluted mixture, as well as reduced soot or particulates.
In another embodiment, when the engine is operated in the controlled self-ignition mode, a mixture of fuel and air supplied for the combustion is lean. The chemical kinetics of a lean mixture proceed much slower than those of a stoichiometric or rich mixture, so that the pressure rise per crank-angle degree is reduced. Also, the low amount of fuel causes the load, hence the maximum cylinder pressure level, to reduce, and therefore, in addition to avoiding knocking, the noise, vibration & harshness (NVH) level will be reduced.
In yet another embodiment, wherein the controlled self-ignition mode comprises a plurality of controlled combustions without spark ignition, in connection to which the mixture of fuel and air is made gradually richer. Thereby, when engine is operated so as to provide a plurality of controlled combustions without spark ignition, for each consecutive combustion, (consecutive firing cylinder), the fuel amount is increased. This means that, during a start of a hot engine where the air in an inlet manifold has been heated by the engine, an adaptation is made to the gradual entry during the starting sequence of cooler air into the inlet manifold, so that the fuel mixture can gradually become richer without the risk of knocking. Also, for each combusting cycle, the engine speed increases causing the auto-ignition timing to be delayed and the pressure rise per crankangle to be decreased, so that for each inducted air volume per engine cycle, the pressure in the manifold will reduce. This will also contribute to the allowance of a gradual increase of the fuel amount supplied, which in turn allows a quicker torque increase at engine start.
Also, the number of controlled combustions without spark ignition is at least partly dependent upon the temperature of a part of the engine at the start procedure, preferably the temperature before the starting sequence. Thereby, the number of controlled combustions without spark ignition can be at least partly dependent upon the temperature before the starting sequence of a cooling fluid of the engine. As an alternative, or additionally, the number of controlled combustions without spark ignition can be at least partly dependent upon the temperature before the start sequence of air in an inlet manifold of the engine.
In another exemplary embodiment, after the controlled self-ignition mode, the engine is operated in a spark ignition (SI) mode. Additionally, before the controlled self-ignition mode, a throttle valve of the engine is controlled so as to assume a not fully open position, which can be a position in which the throttle valve is less than twenty percent open, preferably a fully closed position. In any case, preferably, before the controlled self-ignition mode, the throttle valve is controlled so that there is a substantial pressure drop over the throttle valve. Additionally, at least one fuel injector of the engine can be controlled so that, when the throttle valve is in the not fully open position, an amount of fuel injected is equal to or less than a fuel injection threshold value, which, in one embodiment, may correspond to no injection of fuel. This means that the throttle can be closed during the first couple of cylinder compressions of the engine without fuel injection. Thereby, the air captured in the manifold will expand, hence the temperature and pressure of the air will drop. Both temperature and pressure affect the controlled auto-ignition timing and by reducing the air temperature and pressure, the controlled auto-ignition can be phased later in the cycles, which reduces the maximum pressure.
Preferably, an amount of throttled cylinder compressions, during which the throttle valve is controlled so as to assume a not fully open position, is dependent upon the temperature in an inlet manifold of the engine. Thereby, preferably the amount of throttled cylinder compressions is increased with increased temperature in the inlet manifold. In this way, the controlled auto-ignition timing is always kept within an acceptable crank-angle interval round piston TDC. The amount of throttled cylinder compressions can also be dependent upon the pressure in the inlet manifold.
As mentioned above, the invention is especially advantageous during “hot” starts, which means that, preferably, the inventive start procedure, including the engine being operated in the controlled self-ignition mode, is carried out if at least a part of the engine, e.g. an engine cooling fluid or the material of an engine component, has a temperature that is higher than a threshold temperature. Alternatively, or in addition, the inventive start procedure can be dependent on the air in an inlet manifold of the engine having a temperature that is higher than an inlet manifold air threshold temperature. Also, alternatively, or in addition, the start procedure can be dependent on the pressure in the inlet manifold of the engine being higher than an inlet manifold pressure threshold value.